Several radioimmunoassay methods for serum 17β-estradiol are presently in use. Methods that have been reported for the determination of 17β-estradiol require preliminary extraction of the steroid and, in some cases, chromatographic separation. Ovulation induction in anovulatory women can be accomplished by the serial administration of gonadotropins (Pergonal). Rapid and accurate determinations of serum 17β-estradiol are necessary to adjust the daily gonadotropin dose. In order to meet this requirement, a direct and rapid radioimmunoassay for serum 17β-estradiol was developed in our laboratory that allows the determination of 17β-estradiol levels in up to fifty samples within 3 to 4 h without the need for prior extraction or purification.